A&A 369, 339–363 (2001) DOI: 10.1051/0004-6361:20010085 c ESO 2001
Astronomy
&
Astrophysics
GAIA: Composition, formation and evolution of the Galaxy
M. A. C. Perryman1, K. S. de Boer2, G. Gilmore3, E. Høg4, M. G. Lattanzi5, L. Lindegren6, X. Luri7, F. Mignard8, O. Pace9, and P. T. de Zeeuw10
1
Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2
Sternwarte Univ. Bonn, Auf dem Hugel 71, 53121 Bonn, Germany
3 University of Cambridge, Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK 4
Copenhagen University Observatory, Juliane Maries Vej 30, 2100 OE Copenhagen, Denmark
5 Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy 6
Lund Observatory, Box 43, 22100 Lund, Sweden
7
Universitat de Barcelona, Departament d’Astronomia i Meteorologia, Avda Diagonal 647, 08028 Barcelona, Spain
8
Observatoire de la Cˆote d’Azur, CERGA, Avenue Copernic, 06130 Grasse, France
9 Future Projects Division of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands 10
Sterrewacht, Jan Hendrik Oort Building, Postbus 9513, 2300 RA Leiden, The Netherlands Received 1 November 2000 / Accepted 5 January 2001
Abstract. The GAIA astrometric mission has recently been approved as one of the next two “cornerstones” of
ESA’s science programme, with a launch date target of not later than mid-2012. GAIA will provide positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy (and into the Local Group), amounting to about 1 percent of the Galactic stellar population. GAIA’s main scientific goal is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. It will advance questions such as when the stars in our Galaxy formed, when and how it was assembled, and its distribution of dark matter. The survey aims for completeness to V = 20 mag, with accuracies of about 10 µas at 15 mag. Combined with astrophysical information for each star, provided by on-board multi-colour photometry and (limited) spectroscopy, these data will have the precision necessary to quantify the early formation, and subsequent dynamical, chemical and star formation evolution of our Galaxy. Additional products include detection and orbital classification of tens of thousands of extra-Solar planetary systems, and a comprehensive survey of some 105−106 minor bodies in our Solar System, through
galaxies in the nearby Universe, to some 500 000 distant quasars. It will provide a number of stringent new tests of general relativity and cosmology. The complete satellite system was evaluated as part of a detailed technology study, including a detailed payload design, corresponding accuracy assesments, and results from a prototype data reduction development.
Key words. instrumentation: miscellaneous – space vehicles: instruments – astrometry – galaxy: general –
techniques: photometric – techniques: radial velocities
1. Introduction
Understanding the details of the Galaxy in which we live is one of the great intellectual challenges embraced by modern science. Our Galaxy contains a complex mix of stars, planets, interstellar gas and dust, radiation, and the ubiquitous dark matter. These components are widely distributed in age (reflecting their birth rate), in space (reflecting their birth places and subsequent motions), on orbits (determined by the gravitational force generated by their own mass), and with complex distributions of Send offprint requests to: M. A. C. Perryman,
e-mail: mperryma@astro.estec.esa.nl
chemical element abundances (determined by the past his-tory of star formation and gas accretion).
Astrophysics has now developed the tools to mea-sure these distributions in space, kinematics, and chemical abundance, and to interpret the distribution functions to map, and to understand, the formation, structure, evolu-tion, and future of our entire Galaxy. This potential under-standing is also of profound significance for quantitative studies of the high-redshift Universe: a well-studied nearby template galaxy would underpin the analysis of unresolved galaxies with other facilities, and at other wavelengths.
representative, part of the Galaxy; (ii) quantification of the present spatial structure, from distances; (iii) knowl-edge of the three-dimensional space motions, to determine the gravitational field and the stellar orbits. Astrometric measurements uniquely provide model-independent dis-tances and transverse kinematics, and form the basis of the cosmic distance scale. Complementary radial velocity and photometric information are required to complete the kinematic and astrophysical information about the indi-vidual objects observed.
Photometry, with appropriate astrometric and astro-physical calibration, gives a knowledge of extinction, and hence, combined with astrometry, provides intrinsic lumi-nosities, spatial distribution functions, and stellar chem-ical abundance and age information. Radial velocities complete the kinematic triad, allowing determination of dynamical motions, gravitational forces, and the distribu-tion of invisible mass. The GAIA mission will provide all this information.
Even before the end of the Hipparcos mission, a pro-posal for an ambitious follow-on space astrometry ex-periment was submitted to ESA (Roemer: Høg 1993; Lindegren et al. 1993a; Høg & Lindegren 1994). The idea of using CCDs as a modulation detector behind a grid (Høg & Lindegren 1993), similar to Hipparcos, was re-placed by the more powerful option adopted for Roemer (Høg 1993) where CCDs measure the direct stellar im-ages in time-delayed integration (TDI) mode in the scan-ning satellite. A more ambitious interferometric mission, GAIA, was proposed and subsequently recommended as a cornerstone mission of the ESA science programme by the Horizon 2000+ Survey Committee in 1994 (Battrick 1994). The GAIA proposal demonstrated that accuracies of 10 µas at 15 mag were achievable using a small inter-ferometer (Lindegren et al. 1993b; Lindegren & Perryman 1996).
The European scientific community and ESA have now completed a detailed study of the science case and in-strument design, identifying a number of further improve-ments, including reverting to full-aperture telescopes (Høg 1995a; Høg 1995b). The results demonstrate that unique and fundamental advances in astrophysics are technically achievable on the proposed time-scales, and within a bud-get profile consistent with the current ESA cornerstone mission financial envelope.
GAIA will be a continuously scanning spacecraft, accu-rately measuring one-dimensional coordinates along great circles in two simultaneous fields of view, separated by a well-known angle. The payload utilises a large but feasible CCD focal plane assembly, passive thermal control, natu-ral short-term instrument stability due to the Sun shield and the selected orbit, and a robust payload design. The telescopes are of moderate size, with no specific manufac-turing complexity. The system fits within a dual-launch Ariane 5 configuration, without deployment of any pay-load elements. The study identifies a “Lissajous” orbit at L2 as the preferred operational orbit, from where about 1 Mbit of data per second is returned to the single ground
station throughout the 5-year mission. A comprehensive accuracy assessment has validated the proposed payload and the subsequent data reduction.
This paper provides a summary of the key features of the improved GAIA design, and the resulting scientific case, evaluated during the recent study phase (ESA 2000; see also http://astro.estec.esa.nl/GAIA). A compar-ison between the scientific goals of GAIA, and other post-Hipparcos space astrometry missions, is given in Sect. 8.
2. Scientific goals
2.1. Structure and dynamics of the galaxy
The primary objective of the GAIA mission is to observe the physical characteristics, kinematics and distribution of stars over a large fraction of the volume of our Galaxy, with the goal of achieving a full understanding of its dy-namics and structure, and consequently its formation and history (see, e.g., Gilmore et al. 1989; Majewski 1993; Ibata et al. 1997; Wyse et al. 1997; de Zeeuw 1999; as well as extensive details of the scientific case given in ESA 2000). An overview of the main Galaxy components and sub-populations is given in Table 1, together with require-ments on astrometric accuracy and limiting magnitude.
2.2. The star formation history of our Galaxy
A central element the GAIA mission is the determination of the star formation histories, as described by the tempo-ral evolution of the star formation rate, and the cumula-tive numbers of stars formed, of the bulge, inner disk, Solar neighbourhood, outer disk and halo of our Galaxy (e.g. Hernandez et al. 2000). Given such information, together with the kinematic information from GAIA, and comple-mentary chemical abundance information, again primarily from GAIA, the full evolutionary history of the Galaxy is determinable (e.g. Freeman 1993; Gilmore 1999).
Table 1. Some Galactic kinematic tracers, with corresponding limiting magnitudes and required astrometric accuracy. For
various tracers (Col. 1), Cols. 2–6 indicate relevant values of parameters leading to the typical range of V magnitudes over which the populations must be sampled (Cols. 7–8). These results demonstrate that the faint magnitude limit of GAIA is essential for probing the different Galaxy populations, while the astrometry accuracies in Cols. 9–12 demonstrate that GAIA will meet the scientific goals (based on Gilmore & Høg 1995)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
Tracer MV ` b d AV V1 V2 T σµ1 σµ01 σ
0 π1
mag deg deg kpc mag mag mag km s−1 µas/yr – – Bulge: gM −1 0 < 20 8 2−10 15 20 100 10 0.01 0.10 HB +0.5 0 < 20 8 2−10 17 20 100 20 0.01 0.20 MS turnoff +4.5 1 −4 8 0−2 19 21 100 60 0.02 0.6 Spiral arms: Cepheids −4 all < 10 10 3−7 14 18 7 5 0.03 0.06 B–M Supergiants −5 all < 10 10 3−7 13 17 7 4 0.03 0.05 Perseus arm (B) −2 140 < 10 2 2−6 12 16 10 3 0.01 0.01 Thin disk: gK −1 0 < 15 8 1−5 14 18 40 6 0.01 0.06 gK −1 180 < 15 10 1−5 15 19 10 8 0.04 0.10
Disk warp (gM) −1 all < 20 10 1−5 15 19 10 8 0.04 0.10
Disk asymmetry (gM) −1 all < 20 20 1−5 16 20 10 15 0.14 0.4
Thick disk: Miras, gK −1 0 < 30 8 2 15 19 50 10 0.01 0.10 HB +0.5 0 < 30 8 2 15 19 50 20 0.02 0.20 Miras, gK −1 180 < 30 20 2 15 21 30 25 0.08 0.65 HB +0.5 180 < 30 20 2 15 19 30 60 0.20 1.5 Halo: gG −1 all < 20 8 2−3 13 21 100 10 0.01 0.10 HB +0.5 all > 20 30 0 13 21 100 35 0.05 1.4 Gravity,KZ: dK +7−8 all all 2 0 12 20 20 60 0.01 0.16 dF8-dG2 +5−6 all all 2 0 12 20 20 20 0.01 0.05
Globular clusters (gK) +1 all all 50 0 12 21 100 10 0.01 0.10
internal kinematics (gK) +1 all all 8 0 13 17 15 10 0.02 0.10
Satellite orbits (gM) −1 all all 100 0 13 20 100 60 0.3 8
2.3. Stellar astrophysics
GAIA will provide distances of unprecedented accuracy for all types of stars of all stellar populations, even those in the most rapid evolutionary phases which are very sparsely represented in the Solar neighbourhood. All parts of the Hertzsprung–Russell diagram will be comprehen-sively calibrated, from pre-main sequence stars to white dwarfs and all transient phases; all possible masses, from brown dwarfs to the most massive O stars; all types of variable stars; all possible types of binary systems down to brown dwarf and planetary systems; all standard dis-tance indicators, etc. This extensive amount of data of extreme accuracy will stimulate a revolution in the ex-ploration of stellar and Galactic formation and evolution,
and the determination of the cosmic distance scale (cf. Lebreton 2000).
2.4. Variability
period-luminosity relationships across a wide range of stellar parameters including metallicity. A systematic variability search will also allow identification of stars in short-lived but key stages of stellar evolution, such as the helium core flash and the helium shell thermal pulses and flashes. Prompt processing will identify many targets for follow-up ground-based studies. Estimated numbers are highly uncertain, but suggest some 18 million variable stars in total, including 5 million “classic” periodic vari-ables, 2–3 million eclipsing binaries, 2000–8000 Cepheids, 60 000–240 000 δ Scuti variables, 70 000 RR Lyrae, and 140 000–170 000 Miras (Eyer & Cuypers 2000).
2.5. Binaries and multiple stars
A key scientific issue regarding double and multiple star formation is the distribution of mass-ratios q. For wide pairs (>0.5 arcsec) this is indirectly given through the distribution of magnitude differences. GAIA will provide a photometric determination of the q-distribution down to q∼ 0.1, covering the expected maximum around q ∼ 0.2. Furthermore, the large numbers of (“5-year”) astrometric orbits, will allow derivation of the important statistics of the very smallest (brown dwarf) masses as well as the detailed distribution of orbital eccentricities (S¨oderhjelm 1999).
GAIA is extremely sensitive to non-linear proper mo-tions. A large fraction of all astrometric binaries with pe-riods from 0.03–30 years will be immediately recognized by their poor fit to a standard single-star model. Most will be unresolved, with very unequal mass-ratios and/or magnitudes, but in many cases a photocentre orbit can be determined. For this period range, the absolute and relative binary frequency can be established, with the im-portant possibility of exploring variations with age and place of formation in the Galaxy. Some 10 million bina-ries closer than 250 pc should be detected, with very much larger numbers still detectable out to 1 kpc and beyond.
2.6. Brown dwarfs and planetary systems
Sub-stellar companions can be divided in two classes: brown dwarfs and planets. There exist three major gene-sis indicators that can help classify sub-stellar objects as either brown dwarfs or planets: mass, shape and align-ment of the orbit, and composition and thermal structure of the atmosphere. Mass alone is not decisive. The ability to simultaneously and systematically determine planetary frequency and distribution of orbital parameters for the stellar mix in the Solar neighbourhood is a fundamen-tal contribution that GAIA will uniquely provide. Any changes in planetary frequency with age or metallicity will come from observations of stars of all ages.
An isolated brown dwarf is typically visible only at ages <1 Gyr because of their rapidly fading lu-minosity with time. However, in a binary system, the mass is conserved, and the gravitational effects on a
main-sequence secondary remain observable over much longer intervals. GAIA will have the power to investi-gate the mass-distribution of brown-dwarf binaries with 1–30 year periods, of all ages, through analysis of the as-trometric orbits.
There are a number of techniques which in principle al-low the detection of extra-Solar planetary systems: these include pulsar timing, radial velocity measurements, as-trometric techniques, transit measurements, microlensing, and direct methods based on high-angular resolution in-terferometric imaging. A better understanding of the con-ditions under which planetary systems form and of their general properties requires sensitivity to low mass planets (down to ∼10 M⊕), characterization of known systems (mass, and orbital elements), and complete samples of planets, with useful upper limits on Jupiter-mass planets out to several AU from the central star (Marcy & Butler 1998; Perryman 2000).
Astrometric measurements good to 2–10 µas will con-tribute substantially to these goals, and will complement the ongoing radial velocity measurement programmes. Although SIM will be able to study in detail targets de-tected by other methods, including microlensing, GAIA’s strength will be its discovery potential, following from the astrometric monitoring of all of the several hundred thou-sand bright stars out to distances of ∼200 pc (Lattanzi et al. 2000).
2.7. Solar system
Solar System objects present a challenge to GAIA because of their significant proper motions, but they promise a rich scientific reward. The minor bodies provide a record of the conditions in the proto-Solar nebula, and their properties therefore shed light on the formation of planetary systems. The relatively small bodies located in the main aster-oid belt between Mars and Jupiter should have experi-enced limited thermal evolution since the early epochs of planetary accretion. Due to the radial extent of the main belt, minor planets provide important information about the gradient of mineralogical composition of the early planetesimals as a function of heliocentric distance. It is therefore important for any study of the origin and evolu-tion of the Solar system to investigate the main physical properties of asteroids including masses, densities, sizes, shapes, and taxonomic classes, all as a function of loca-tion in the main belt and in the Trojan clouds.
The possibility of determining asteroid masses relies on the capability of measuring the tiny gravitational per-turbations that asteroids experience in case of a mutual close approach. At present only about 10 asteroid masses are known, mostly with quite poor accuracy. Asteroid-asteroid encounters have been modelled, and show that GAIA will allow more than 100 asteroid masses to be de-termined accurately.
The GAIA photometry will be much more reliable than most data presently available. The colour indices will pro-vide a taxonomic classification for the whole sample of observed asteroids.
For direct orbit determinations of known asteroids, preliminary simulations have been performed in which the covariance matrices of the orbital elements of more than 6000 asteroids were computed using both the whole set of astrometric observations collected from ground-based telescopes since 1895 through 1995, as well as a set of simulated observations carried out by GAIA, computed by considering a 5 year lifetime of the mission, and present in-strument performances. Another set of simulated ground-based observations covering the period 1996–2015 were also performed. For the known asteroids the predicted ephemeris errors based on the GAIA observations alone 100 years after the end of the mission are more than a factor 30 better than the predicted ephemeris errors cor-responding to the whole set of past and future ground-based observations. In other words, after the collection of the GAIA data, all the results of more than one cen-tury of ground-based asteroid astrometry will be largely superseded.
In addition to known asteroids, GAIA will discover a very large number, of the order of 105or 106new objects,
depending on the uncertainties on the extrapolations of the known population. It should be possible to derive pre-cise orbits for many of the newly discovered objects, since each of them will be observed many times during the mis-sion lifetime. These will include a large number of near-Earth asteroids. The combination of on-board detection, faint limiting magnitude, observations at small Sun-aspect angles, high accuracy in the instantaneous angular veloc-ity (0.25 mas s−1), and confirmation from successive field transits, means that GAIA will provide a detailed census of Atens, Apollos and Amors, extending as close as 0.5 AU to the Sun, and down to diameters of about 260–590 m at 1 AU, depending on albedo and observational geometry.
2.8. Galaxies, quasars, and the reference frame
GAIA will not only provide a representative census of the stars throughout the Galaxy, but it will also make unique contributions to extragalactic astronomy (Table 2). These include the structure, dynamics and stellar populations in the Local Group, especially the Magellanic Clouds, M 31 and M 33, the space motions of Local Group galaxies, a multi-colour survey of galaxies (Vaccari 2000), and stud-ies of supernovae (Høg et al. 1999b), galactic nuclei, and quasars.
2.9. The radio/optical reference frame
The International Celestial Reference System (ICRS) is realized by the International Celestial Reference Frame (ICRF) consisting of 212 extragalactic radio-sources with an rms uncertainty in position between 100 and 500 µas.
The extension of the ICRF to visible light is represented by the Hipparcos Catalogue. This has rms uncertainties estimated to be 0.25 mas yr−1 in each component of the spin vector of the frame, and 0.6 mas in the com-ponents of the orientation vector at the catalogue epoch, J1991.25. The GAIA catalogue will permit a definition of the ICRS more accurate by one or two orders of magni-tude than the present realizations (e.g. Feissel & Mignard 1998; Johnston & de Vegt 1999).
The spin vector can be determined very accurately by means of the many thousand faint quasars picked up by the astrometric and photometric survey. Simulations using realistic quasar counts, conservative estimates of intrin-sic source photocentric instability, and realistic interven-ing gravitational lensinterven-ing effects, show that an accuracy of better than 0.4 µas yr−1 will be reached in all three components of the spin vector.
For the determination of the frame orientation, the only possible procedure is to compare the positions of the radio sources in ICRF (and its extensions) with the po-sitions of their optical counterparts observed by GAIA. The number of such objects is currently less than 300 and the error budget is dominated by the uncertainties of the radio positions. Assuming current accuracies for the radio positions, simulations show that the GAIA frame orien-tation will be obtained with an uncertainty of ∼60 µas in each component of the orientation vector. The actual result by the time of GAIA may be significantly better, as the number and quality of radio positions for suitable objects are likely to increase with time.
The Sun’s absolute velocity with respect to a cosmo-logical reference frame causes the dipole anisotropy of the cosmic microwave background. The Sun’s absolute accel-eration can be measured astrometrically: it will result in the apparent proper motion of quasars. The acceleration of the Solar System towards the Galactic centre causes the aberration effect to change slowly. This leads to a slow change of the apparent positions of distant celestial ob-jects, i.e., to an apparent proper motion. For a Solar veloc-ity of 220 km s−1and a distance of 8.5 kpc to the Galactic centre, the orbital period of the Sun is∼250 Myr, and the Galactocentric acceleration has the value 0.2 nm s−2, or 6 mm s−1yr−1. A change in velocity by 6 mm s−1 causes a change in aberration of the order of 4 µas. The apparent proper motion of a celestial object caused by this effect al-ways points towards the direction of the Galactic centre. Thus, all quasars will exhibit a streaming motion towards the Galactic centre of this amplitude.
2.10. Fundamental physics
The reduction of the Hipparcos data necessitated the in-clusion of stellar aberration up to terms in (v/c)2, and the
Table 2. Local Group galaxies potentially accessible to GAIA. E(B− V ) indicates the foreground reddening, and (m − M)0
is the true distance modulus. Vlim is the brightest star in the galaxy. µvt−vr is the estimated proper motion, assuming the
transverse velocity equals the observed radial velocity. * denotes observed values
Galaxy l b E(B− V ) (m− M)0 Distance Vlim N (stars) Vr µvt−vr
(◦) (◦) (mag) (mag) (kpc) (mag) (V < 20) (helio) (µas/yr) WLM 75.9 −73.6 0.02± 0.01 24.83± 0.08 925± 40 16.5 ∼500 −116 26 NGC 55 332.7 −75.7 0.03± 0.02 25.85± 0.20 1480± 150 15.0 10’s 129 18 IC 10 119.0 −3.3 0.87± 0.12 24.58± 0.12 825± 50 15.0 10’s −344 83 NGC 147 119.8 −14.3 0.18± 0.03 24.30± 0.12 725± 45 18.5 10’s −193 56 And III 119.3 −26.2 0.05± 0.02 24.40± 0.10 760± 40 20 60 NGC 185 120.8 −14.5 0.19± 0.02 23.96± 0.08 620± 25 20 −202 69 NGC 205 120.7 −21.7 0.04± 0.02 24.56± 0.08 815± 35 20 −241 62 M 32 121.2 −22.0 0.08± 0.03 24.53± 0.08 805± 35 16 ∼ 104 −205 54 M 31 121.2 −21.6 0.08 24.43 770 15 104 −297 81 And I 121.7 −24.9 0.04± 0.02 24.53± 0.10 805± 40 21.7 SMC 302.8 −44.3 0.08 18.82 58 12 > 106 158 900∗ Sculptor 287.5 −83.2 0.02± 0.02 19.54± 0.08 79± 4 16.0 100’s 110 360∗ LGS 3 126.8 −40.9 0.08± 0.03 24.54± 0.15 810± 60 −277 72 IC 1613 129.8 −60.6 0.03± 0.02 24.22± 0.10 700± 35 17.1 100’s −234 71 And II 128.9 −29.2 0.08± 0.02 23.6± 0.4 525± 110 20 M 33 133.6 −31.3 0.08 24.62 840 15 > 104 −181 46 Phoenix 272.2 −68.9 0.02± 0.01 23.24± 0.12 445± 30 17.9 ∼ 102 56 27 Fornax 237.1 −65.7 0.03± 0.01 20.70± 0.12 138± 8 14 100’s 53 81 EGB 0427+63 144.7 −10.5 0.30± 0.15 25.6± 0.7 1300± 700 −99 16 LMC 280.5 −32.9 0.06 18.45 49 12 > 107 278 1150∗ Carina 260.1 −22.2 0.04± 0.02 20.03± 0.09 101± 5 18 ∼ 103 229 478 Leo A 196.9 +52.4 0.01± 0.01 24.2± 0.3 690± 100 20 6 Sextans B 233.2 +43.8 0.01± 0.02 25.64± 0.15 1345± 100 19.0 10’s 301 47 NGC 3109 262.1 +23.1 0.04± 0.02 25.48± 0.25 1250± 165 403 68 Antlia 263.1 +22.3 0.05± 0.03 25.46± 0.10 1235± 65 361 62 Leo I 226.0 +49.1 0.01± 0.01 21.99± 0.20 250± 30 19 10’s 168 142 Sextans A 246.2 +39.9 0.03± 0.02 25.75± 0.15 1440± 110 17.5 10’s 324 48 Sextans 243.5 +42.3 0.03± 0.01 19.67± 0.08 86± 4 230 564 Leo II 220.2 +67.2 0.02± 0.01 21.63± 0.09 205± 12 18.6 100’s 90 95 GR 8 310.7 +77.0 0.02± 0.02 25.9± 0.4 1510± 330 18.7 10’s 214 28 Ursa Minor 105.0 +44.8 0.03± 0.02 19.11± 0.10 66± 3 16.9 100’s −209 1000∗ Draco 86.4 +34.7 0.03± 0.01 19.58± 0.15 82± 6 17 100’s −281 1000∗ Sagittarius 5.6 −14.1 0.15± 0.03 16.90± 0.15 24± 2 14 > 104 140 2100∗ SagDIG 21.1 −16.3 0.22± 0.06 25.2± 0.3 1060± 160 −77 16 NGC 6822 25.3 −18.4 0.26± 0.04 23.45± 0.15 490± 40 −57 25 DDO 210 34.0 −31.3 0.06± 0.02 24.6± 0.5 800± 250 18.9 10’s −137 36 IC 5152 343.9 −50.2 0.01± 0.02 26.01± 0.25 1590± 200 124 16 Tucana 322.9 −47.4 0.00± 0.02 24.73± 0.08 880± 40 18.5 10’s Pegasus 94.8 −43.5 0.02± 0.01 24.90± 0.10 955± 50 20 −183 40
the opportunity to test a number of parameters of general relativity in new observational domains, and with much improved precision.
The dominant relativistic effect in the GAIA measure-ments is gravitational light bending, quantified by, and
Table 3. Light deflection by masses in the Solar System. The monopole effect dominates, and is summarized in the left columns
for grazing incidence and for typical values of the angular separation. Columns χmin and χmax give results for the minimum
and maximum angles accessible to GAIA. J2 is the quadrupole moment. The magnitude of the quadrupole effect is given for
grazing incidence, and for an angle of 1◦. For GAIA this applies only to Jupiter and Saturn, as it will be located at L2, with minimum Sun/Earth avoidance angle of 35◦
Object Monopole term Quadrupole term
Grazing χmin χ = 45◦ χ = 90◦ χmax J2 Grazing χ = 1◦
(µas) (µas) (µas) (µas) (µas) (µas) (µas)
Sun 1 750 000 13 000 10 000 4100 2100 ≤ 10−7 0.3 –
Earth 500 3 2.5 1.1 0 0.001 1 –
Jupiter 16 000 16 000 2.0 0.7 0 0.015 500 7 10−5
Saturn 6000 6000 0.3 0.1 0 0.016 200 3 10−6
photons, an effect probing the time-time component of the metric tensor. Light deflection depends on both the time-space and time-space-time-space components. It has been observed on distance scales of 109−1021m, and on mass scales from
1−1013M
. GAIA will extend the domain of observations
by two orders of magnitude in length, and six orders of magnitude in mass.
Table 3 gives the magnitude of the deflection for the Sun and the major planets, at different values of the angu-lar separation χ, for both monopole and quadrupole terms. While χ is never smaller than 35◦for the Sun (a constraint from GAIA’s orbit), grazing incidence is possible for the planets. With the astrometric accuracy of a few µas, the magnitude of the expected effects is considerable for the Sun, and also for observations near planets. The GAIA astrometric residuals can be tested for any discrepancies with the prescriptions of general relativity. Detailed anal-yses indicate that the GAIA measurements will provide a precision of about 5 10−7for γ, based on multiple observa-tions of∼107stars with V < 13 mag at wide angles from
the Sun, with individual measurement accuracies better than 10 µas.
Recent developments in cosmology (e.g. inflationary models) and elementary-particle physics (e.g. string ory and Kaluza-Klein theories), consider scalar-tensor the-ories as plausible alternatives to general relativity. A large class of such theories contain an attractor mechanism to-wards general relativity in a cosmological sense; if this is how the Universe is evolving, then today we can expect discrepancies of the order of |γ − 1| ∼ 10−7−10−5 de-pending on the theory. This kind of argument provides a strong motivation for any experiments able to reach these accuracies.
GAIA will observe and discover several hundred thou-sand minor planets during its five year mission. Most of these will belong to the asteroidal main belt, with small orbital eccentricity and semi-major axes close to 3 AU. The members of the Apollo and Aten groups, which are all Earth-orbit crossers, will include objects with semi-major axes of the order of 1 AU and eccentricities as large as 0.9. The Amor group have perihelia between 1–1.3 AU, and approach the Earth but do not cross its orbit.
Table 4. Perihelion precession due to general relativity and
the Solar quadrupole moment for a few representative objects. a = semi-major axis; e = eccentricity; GR = perihelion pre-cession in mas/yr due to general relativity; J2 = perihelion
precession in mas/yr due to the Solar quadrupole moment (as-suming J2= 10−6)
Body a e GR J2
(AU) (mas/yr) (mas/yr)
Mercury 0.39 0.21 423 1.24
Asteroids 2.7 0.1 3.4 0.001
1566 Icarus 1.08 0.83 102 0.30
5786 Talos 1.08 0.83 102 0.30
3200 Phaeton 1.27 0.89 103 0.40
Relativistic effects and the Solar quadrupole cause the orbital perihelion of a main belt asteroid to precess at a rate about seven times smaller than for Mercury in rate per revolution, although more than a hundred times in absolute rate.
Three cases of Earth-crossing asteroids are considered in Table 4 giving perihelia precession larger than Mercury, due to a favorable combination of distance and eccentric-ity. The diameters are of the order of 1 km for Icarus and Talos and 4 km for Phaeton. Observed at a geocen-tric distance of 1 AU, these objects have a magnitude be-tween V = 15−17 mag and an angular diameter of 4 mas and 1 mas respectively. Thus the astrometric measure-ments will be of good quality, virtually unaffected by the finite size of the source. A determination of λ with an ac-curacy of 10−4 is a reasonable goal, with a value closer to 10−5 probably attainable from the statistics on sev-eral tens of planets. An independent determination of the Solar quadrupole moment J2 requires good sampling in
a(1− e2), and one can expect a result better than 10−7.
from the binary pulsar PSR 1913+16. Since this is a sta-tistical upper limit, any improvement in our knowledge of the white dwarf luminosity function of the Galactic disk will translate into a more stringent upper bound for
˙
G/G. Since GAIA will detect numerous white dwarfs at low luminosities, present errors can be reduced by a fac-tor of roughly 5. If a reliable age of the Solar neighbour-hood independent of the white dwarf luminosity function is determinable, the upper limit could be decreased to 10−12−10−13 yr−1.
2.11. Summary
With a census of the accurate positions, distances, space motions (proper motions and radial velocities), and pho-tometry of all approximately one billion objects complete to V = 20 mag, GAIA’s scientific goals are immense, and can be broadly classified as follows:
The Galaxy: origin and history of our Galaxy; tests of hierarchical structure formation theories; star formation history; chemical evolution; inner bulge/bar dynamics; disk/halo interactions; dynamical evolution; nature of the warp; star cluster disruption; dynamics of spiral structure; distribution of dust; distribution of invisible mass; detec-tion of tidally disrupted debris; Galaxy rotadetec-tion curve; disk mass profile.
Star formation and evolution: in situ luminosity func-tion; dynamics of star forming regions; luminosity function for pre-main sequence stars; detection and categorization of rapid evolutionary phases; complete and detailed local census down to single brown dwarfs; identification/dating of oldest halo white dwarfs; age census; census of binaries and multiple stars.
Distance scale and reference frame: parallax calibra-tion of all distance scale indicators; absolute luminosities of Cepheids; distance to the Magellanic Clouds; definition of the local, kinematically non-rotating metric.
Local Group and beyond: rotational parallaxes for Local Group galaxies; kinematical separation of stellar populations; galaxy orbits and cosmological history; zero proper motion quasar survey; cosmological acceleration of Solar System; photometry of galaxies; detection of super-novae.
Solar System: deep and uniform detection of minor planets; taxonomy and evolution; inner Trojans; Kuiper Belt Objects; near-Earth asteroids; disruption of Oort Cloud.
Extra-Solar planetary systems: complete census of large planets to 200–500 pc; masses; orbital characteristics of several thousand systems; relative orbital inclinations of multiple systems.
Fundamental physics: γ to ∼5 10−7; β to 3 10−4−3 10−5; Solar J2 to 10−7−10−8; G/G to˙
10−12−10−13 yr−1; constraints on gravitational wave energy for 10−12 < f < 4 10−9 Hz; constraints on ΩM
and ΩΛ from quasar microlensing.
Examples of specific objects: 106−107 resolved
galax-ies; 105 extragalactic supernovae; 500 000 quasars;
105−106 (new) Solar System objects; >
∼50 000 brown dwarfs; 30 000 extra-Solar planets; 200 000 disk white dwarfs; 200 microlensed events; 107 resolved binaries
within 250 pc.
3. Overall design considerations
Instrument design converges through a consideration of technical feasibility and scientific requirements. The pro-posed GAIA design has arisen from requirements on as-trometric precision (10 µas at 15 mag), completeness to V = 20 mag, the acquisition of radial velocities, the provi-sion of accurate multi-colour photometry for astrophysical diagnostics, and the need for on-board object detection (Mignard 1999; Gilmore et al. 2000).
3.1. Astrometry
A space astrometry mission has a unique capability to perform global measurements, such that positions, and changes in positions caused by proper motion and par-allax, are determined in a reference system consistently defined over the whole sky, for very large numbers of ob-jects. Hipparcos demonstrated that this can be achieved with milliarcsecond accuracy by means of a continuously scanning satellite which observes two directions simultane-ously. With current technology this same principle can be applied with a gain of a factor of more than 100 improve-ment in accuracy, a factor 1000 improveimprove-ment in limiting magnitude, and a factor of 10 000 in the numbers of stars observed.
Measurements conducted by a continuously scanning satellite are optimally efficient, with each photon acquired during a scan contributing to the precision of the resulting astrometric parameters. The over-riding benefit of global astrometry using a scanning satellite is however not effi-ciency but reliability: an accurate instrument calibration is performed naturally, while the interconnection of ob-servations over the celestial sphere provides the rigidity and reference system, immediately connected to an extra-galactic reference system, and a realistic determination of the standard errors of the astrometric parameters. Two individual viewing directions with a wide separation is the fundamental pre-requisite of the payload, since this leads to the determination of absolute trigonometric par-allaxes, and absolute distances, exploiting the method im-plemented for the first time in the Hipparcos mission.
accuracy will be of order (λ/D)× N−1/2 radians. A re-alistic size for non-deployable space instruments is of or-der 2 m. Operating in visible light (λ ∼ 0.5 µm) then gives diffraction features of order λ/D ∼ 0.05 arcsec. To achieve a final astrometric accuracy of 10 µas it is there-fore necessary that the diffraction features are localised to within 1/5000 of their characteristic size. Thus, some 25 million detected photons are needed to overcome the statistical noise, although extreme care will be needed to achieve such precision in practice. The requirement on the number of photons can be satisfied for objects around 15 mag with reasonable assumptions on collecting area and bandwidth. Quantifying the tradeoff between di-lute versus filled apertures, allowing for attainable focal lengths, attainable pixel sizes, component alignment and stability, and data rates, has clearly pointed in the direc-tion of a moderately large filled aperture (as apposed to an interferometric design) as the optical system of choice. The GAIA performance target is 10 µas at 15 mag. Restricting GAIA to a limiting magnitude of 15 mag, or to a subset of all objects down to its detection limit, would provide a reduction in the down-link telemetry rate, but little or no change in the other main aspects of the pay-load design. These are driven simply by the photon noise budget required to reach a 10 µas accuracy at 15 mag. The faint magnitude limit, the ability to meet the adopted sci-entific case, and the number of target objects follow from the accuracy requirement, with no additional spacecraft cost.
3.2. Radial velocity measurements
There is one dominant scientific requirement, as well as two additional scientific motivations, for the acquisition of radial velocities with GAIA: (i) astrometric measure-ments supply only two components of the space motion of the target stars: the third component, radial velocity, is directed along the line of sight, but is nevertheless es-sential for dynamical studies; (ii) measurement of the ra-dial velocity at a number of epochs is a powerful method for detecting and characterising binary systems; (iii) at the GAIA accuracy levels, “perspective acceleration” is at the same time both a complication and an important ob-servable quantity. If the distance between an object and observer changes with time due to a radial component of motion, a constant transverse velocity is observed as a varying transverse angular motion, the perspective accel-eration. Although the effect is generally small, some hun-dreds of thousands of high-velocity stars will have system-atic distance errors if the radial velocities are unknown.
On-board acquisition of radial velocities with GAIA is not only feasible, but is relatively simple, is scientifically necessary, and cannot be readily provided in any other way. In terms of accuracy requirements, faint and bright magnitude regimes can be distinguished. The faint tar-gets will mostly be distant stars, which will be of interest as tracers of Galactic dynamics. The uncertainty in the
tangential component of their space motion will be domi-nated by the error in the parallax. Hence a radial velocity accuracy of'5 km s−1is sufficient for statistical purposes. Stars with V <∼ 15 mag will be of individual interest, and the radial velocity will be useful also as an indicator of multiplicity and for the determination of perspective ac-celeration. The radial velocities will be determined by dig-ital cross-correlation between an observed spectrum and an appropriate template. The present design allows (for red Population I stars of any luminosity class) determina-tion of radial velocities to σv ' 5 km s−1 at V = 18 mag
(e.g. Munari 1999a).
Most stars are intrinsically red, and made even redder by interstellar absorption. Thus, a red spectral region is to be preferred for the GAIA spectrograph. To maximize the radial velocity signal even for metal-poor stars, strong, saturated lines are desirable. Specific studies, and ground-based experience, show that the Ca ii triplet near 860 nm is optimal for radial velocity determination in the greatest number of stellar types.
Ground-based radial velocity surveys are approach-ing the one million-object level. That experience shows the cost and complexity of determining some hundreds of millions of radial velocities is impractical. There is also a substantial additional scientific return in acquiring a large number of measurements, and doing so not only well spaced in time but also, preferably, simultaneously with the astrometric measurements (e.g. variables and multi-ple systems).
3.3. Derivation of astrophysical parameters
The GAIA core science case requires measurement of lumi-nosity, effective temperature, mass, age and composition, in addition to distance and velocity, to optimise under-standing of the stellar populations in the Galaxy and its nearest neighbours. The quantities complementary to the kinematics can be derived from the spectral energy distri-bution of the stars by multi-band photometry and spec-troscopy. Acquisition of this astrophysical information is an essential part of the GAIA payload. A broad-band mag-nitude, and its time dependence, will be obtained from the primary mission data, allowing both astrophysical analy-ses and the critical corrections for residual system chro-maticity. For the brighter stars, the radial.
100 300 500 700 900 1100 wavelength (nm) 0.0 0.2 0.4 0.6 0.8 1.0 response curve CCD#1B F45B F63B F82B F33B 100 300 500 700 900 1100 wavelength (nm) 0.0 0.2 0.4 0.6 0.8 1.0 response curve CCD#1B (G) CCD#2 33 41 47 51 57 67 7578 82 89 Ha
Fig. 1. Filter transmission curves and CCD response curves for the provisional (baseline) broad-band (left) and medium-band
(right) photometric systems
α-elements (at the same accuracy level) will be desirable for mapping Galactic chemical evolution. These require-ments translate into a magnitude accuracy of'0.02 mag for each colour index.
Many photometric systems exist, but none is necessar-ily optimal for space implementation. For GAIA, photom-etry will be required for quasar and galaxy photomphotom-etry, Solar System object classification, etc. Considerable effort has therefore been devoted to the design of an optimum fil-ter system for GAIA (e.g. Høg et al. 1999a; Munari 1999b). The result of this effort is a baseline system, with four broad and eleven medium passbands, covering the near ul-traviolet to the CCD red limit. The filters are summarised in Fig. 1. The 4 broad-band filters are implemented within the astrometric fields, and therefore yield photometry at the same angular resolution (also relevant for chromatic correction), while the 11 medium-band filters are im-plemented within the spectrometric telescope. Both tar-get magnitude limits of 20 mag, as for the astrometric measurements.
3.4. On-board detection
Clear definition and understanding of the selection func-tion used to decide which targets to observe is a crucial scientific issue, strongly driving the final scientific output of the mission. The optimum selection function, and that adopted, is to detect every target above some practical signal level on-board as it enters the focal plane. This has the advantage that the detection will be carried out in the same wave-band, and at the same angular resolution, as the final observations. The focal plane data on all objects down to about 20 mag can then be read out and teleme-tered to ground within system capabilities. All objects, including Solar System objects, variable objects, super-novae, and microlensed sources, are detected using this “astrometric sky mapper”, described in further detail in Sect. 4.3.
Basic angle monitoring device ASTRO-1 focal plane
ASTRO-1 secondary mirror
ASTRO-1 primary mirror ASTRO-2 primary mirror
Spectrometric Instrument (Secondary Reflector & Focal Plane)
Platform Interface (Titanium bipods) Spectrometric Instrument
(Primary and tertiary mirrors)
Common Optical Bench
Wide Field Star sensor
Fig. 2. The payload includes two identical astrometric
in-struments (labelled ASTRO-1 and ASTRO-2) separated by the 106◦ basic angle, as well as a spectrometric instrument (comprising a radial velocity measurement instrument and a medium-band photometer) which share the focal plane of a third viewing direction. All telescopes are accommodated on a common optical bench of the same material, and a basic angle monitoring device tracks any variations in the relative viewing directions of the astrometric fields
4. Payload design
4.1. Measurement principles
The overall design constraints have been investigated in detail in order to optimise the number and optical de-sign of each viewing direction, the choice of wavelength bands, detection systems, detector sampling strategies, basic angle, metrology system, satellite layout, and or-bit (M´erat et al. 1999). The resulting proposed payload design (Fig. 2) consists of:
fields separated by a basic angle of 106◦. Each astromet-ric field comprises an astrometastromet-ric sky mapper, the astro-metric field proper, and a broad-band photometer. Each sky mapper system provides an on-board capability for star detection and selection, and for the star position and satellite scan-speed measurement. The main focal plane assembly employs CCD technology, with about 250 CCDs and accompanying video chains per focal plane, a pixel size 9 µm along scan, TDI (time-delayed integration) op-eration, and an integration time of∼0.9 s per CCD;
(b) an integrated radial velocity spectrometer and pho-tometric instrument, comprising an all-reflective three-mirror telescope of aperture 0.75× 0.70 m2. The field of view is separated into a dedicated sky mapper, the radial velocity spectrometer, and a medium-band photometer. Both instrument focal planes are based on CCD technol-ogy operating in TDI mode;
(c) the opto-mechanical-thermal assembly comprising: (i) a single structural torus supporting all mirrors and fo-cal planes, employing SiC for both mirrors and structure. There is a symmetrical configuration for the two astromet-ric viewing directions, with the spectrometastromet-ric telescope ac-commodated within the same structure, between the two astrometric viewing directions; (ii) a deployable Sun shield to avoid direct Sun illumination and rotating shadows on the payload module, combined with the Solar array as-sembly; (iii) control of the heat injection from the service module into the payload module, and control of the focal plane assembly power dissipation in order to provide an ultra-stable internal thermal environment; (iv) an align-ment mechanism on the secondary mirror for each astro-metric instrument, with micron-level positional accuracy and 200 µm range, to correct for telescope aberration and mirror misalignment at the beginning of life; (v) a per-manent monitoring of the basic angle, but without active control on board.
The accuracy goal is to reach a 10 µas rms positional accuracy for stars of magnitude V = 15 mag. For fainter magnitudes, the accuracy falls to about 20−40 µas at V = 17−18 mag, and to 100−200 µas at V = 20 mag, entirely due to photon statistics. For V < 15 mag, higher accuracy is achieved, but will be limited by systematic ef-fects at about 3−4 µas for V < 10−11 mag. Raw data representing the star profile along scan must be sent to ground. An integral objective of the mission is to provide the sixth astrometric parameter, radial velocity, by mea-suring the Doppler shift of selected spectral lines. Colour information is to be acquired for all observed objects, pri-marily to allow astrophysical analyses, though calibration of the instrument’s chromatic dependence is a key sec-ondary consideration.
The astrometric accuracy can be separated into two independent terms, the random part induced by photo-electron statistics on the localisation process accuracy, and a bias error which is independent of the number of col-lected photons. The random part decreases in an ideal system as N−0.5, where N is the number of detected electrons per star; the bias part is independent of N ,
Table 5. Summary of the scanning law and pointing
require-ments. 0.05 Hz is the maximum frequency that can be identified after measurement post-processing
Parameter Value
Satellite scan axis tilt angle 55◦to the Sun
Scan rate 120 arcsec s−1
Absolute scan rate error 1.2 arcsec s−1(3σ)
Precession rate 0.17 arcsec s−1
Absolute precession rate error 0.1 arcsec s−1(3σ) Absolute pointing error 5 arcmin (3σ) Attitude absolute measurement error 0.001 arcsec (1σ) High-frequency disturbances:
power spectral density at 0.05 Hz ≤ 1000 µas2 Hz−1 for f > 0.05 Hz decreasing as f−2
represents the ultimate capability of the system for bright stars, is limited by payload stability on timescales shorter than those which can be self-calibrated, i.e. shorter than about 5 hours.
GAIA will operate through continuous sky scanning, this mode being optimally suited for a global, survey-type mission with very many targets, and being of proven va-lidity from Hipparcos. The satellite scans the sky accord-ing to a pre-defined pattern in which the axis of rotation (perpendicular to the three viewing directions) is kept at a nominally fixed angle ξ from the Sun, describing a preces-sional motion about the Solar direction at constant speed with respect to the stars. This angle is optimised against satellite Sun shield demands, parallax accuracy, and scan-ning law. Resulting satellite pointing performances are determined from operational and scientific processing re-quirements on ground, and are summarised in Table 5.
A mission length of 5 years is adopted for the satellite design lifetime, which starts at launcher separation and includes the transfer phase and all provisions related to system, satellite or ground segment dead time or outage. A lifetime of 6 years has been used for the sizing of all consumables.
4.2. Optical design
The astrometric telescopes have a long focal length, neces-sary for oversampling the individual images. A pixel size of 9 µm in the along-scan direction was selected, with the 50 m focal length allowing a 6-pixel sampling of the diffraction image along scan at 600 nm. The resulting op-tical system is very compact, fitting into a volume 1.8 m high, and within a mechanical structure adapted to the Ariane 5 launcher. Deployable payload elements have been avoided. System optimisation yields a suitable full pupil of 1.7× 0.7 m2 area with a rectangular shape. Optical
mechanical complexity. The optical configuration is de-rived from a three-mirror anastigmatic design with an in-termediate image. The three mirrors have aspheric sur-faces with limited high-order terms, and each of them is a part of a rotationally symmetric surface. The aperture shapes are rectangular and decentered, while each mirror is slightly tilted and decentered.
The tolerable optical distortion arises from the require-ment that any variation of scale across the field must not cause significant image blurring during TDI operation. The number and size of the CCDs has been determined to match the optical quality locally in the field.
The monochromatic point spread function, Pλ(ξ, η), at
a specific point in the field, is related to the correspond-ing wavefront error map w(x, y) in the pupil plane through the diffraction formula. The overall wavefront error of the telescope is the sum of the errors arising from optical de-sign, alignment, and polishing residuals for the three mir-rors. The design target of λ/50 rms over the whole field corresponds to a Strehl ratio of 0.84 at 500 nm. From anal-ysis performed using the optical design software package Code V, alignment errors can be made negligible (wave front error < λ/70 rms) provided that the mirrors are positioned with an accuracy of about±1 µm. A 5 degree-of-freedom compensation mechanism with this accuracy (not considered to be excessively stringent with piezo-type actuators) is therefore implemented on the secondary re-flector of each of the astrometric telescopes. This allows optimization of the overall optical quality in orbit as a result of on-ground residual alignment errors, and the re-covery of misalignments of the telescope optics which may be induced by launch effects, even if all the mirrors are randomly misaligned by an amplitude ±50 µm in all di-rections. The required wavefront error measurement will be performed on at least three points of the field of view. In practice, the astrometric performance is not strongly dependent on the actual telescope wavefront error, since the effect of aberrations corresponds to first order in an energy loss in the central diffraction peak, which is the only part of the point spread function used for the star localization.
Although the optical design only employs mirrors, diffraction effects with residual (achromatic) aberrations induce a small chromatic shift of the diffraction peak. The chromaticity image displacement depends on position in the field, and on the star’s spectral energy distribution (colour), but not on its magnitude. One purpose of the broad-band photometric measurements within the main field is to provide colour information on each observed ob-ject in the astrometric field to enable this chromaticity bias calibration on ground. Recent developments made on ion beam polishing have shown that polishing errors can be made practically negligible (λ/100 rms obtained on a SiC reflector of about 200 mm diameter). It is therefore likely that the chromatic shift can be reduced below a few tens of µas over the whole field, easing calibration re-quirements. Combination of the satellite Sun shield and internal baffling reduce straylight to negligible levels.
Vout Vdd 2780 pixels µ 2150 pixels (or 1075) 9x27 m2 Airy pattern useful window scan direction
selectable gate phase
gain selection Vreset controlled for windowing parallel clocks operated in TDI serial clocks, controlled for windowing non-useful pixels (to be flushed) useful pixels (to be binned and read-out)
Fig. 3. Operating mode for the astrometric field CCDs. The
location of the star is known from the astrometric sky map-per, combined with the satellite attitude. A window is selected around the star in order to minimise the resulting read-out noise of the relevant pixels
4.3. Astrometric focal plane
The focal plane contains a set of CCDs operating in TDI (time-delayed integration) mode, scanning at the same ve-locity as the spacecraft scanning veve-locity and thus inte-grating the stellar images until they are transferred to the serial register for read out. Three functions are assigned to the focal plane system: (i) the astrometric sky map-per; (ii) the astrometric field, devoted to the astromet-ric measurements; (iii) the broad band photometer, which provides broad-band photometric measurements for each object. The same elementary CCD is used for the entire focal plane, with minor differences in the operating modes depending on the assigned functions.
Fig. 4. Nominal accuracy performance versus magnitude, for
a G2V star. For V < 15 mag the astrometric performance im-proves because the number of detected photons increases until the detector saturation level is reached (V ∼ 11.6 for G2V star). Brighter than this, the performance is practically inde-pendent of magnitude, due to the pre-selection of the number of TDI stages required to avoid saturation
and allow the prediction of the the individual star transits across the main astrometric field with adequate precision for the foreseen windowing mode.
The size of the astrometric field is optimised at sys-tem level to achieve the specified accuracy, with a field of 0.◦5 × 0.◦66. The size of the individual CCD is a compromise between manufacturing yield, distortion, and integration time constraints. The pixel size is a compro-mise between manufacturing feasibility, detection perfor-mances (QE and MTF), and charge-handling capacity: a dimension of 9 µm in the along-scan direction pro-vides full sampling of the diffraction image, and a size of 27 µm in the across-scan direction is compatible with the size of the dimensions of the point spread function and cross scan image motion. In addition, it provides space for implementation of special features for the CCD (e.g. pixel anti-blooming drain) and provides improved charge-handling capacity. Quantitative calculations have demon-strated that the pixel size, TDI smearing, pixel sampling, and point spread function are all matched to system re-quirements. The CCDs are slightly rotated in the focal plane and are individually sequenced in order to compen-sate for the telescope optical distortion. Cross-scan bin-ning of 8 pixels is implemented in the serial register for improvement of the signal-to-noise ratio.
Each individual CCD features specific architecture al-lowing measurement of stars brighter than the normal sat-uration limit of about V = 11−12 mag: selectable gate phases allow pre-selection of the number of TDI stages to be used within a given CCD array. The resulting as-trometric error versus magnitude shows the effect of this discrete selection (Fig. 4).
At the apparent magnitude and integration time lim-its appropriate for GAIA most of the pixel data do not include any useful information. There is a clear trade-off
between reading too many pixels, with associated higher read-noise and telemetry costs, and reading too few, with associated lost science costs. This contributes to the choice of on-board real-time detection, with definition of a win-dow around each source which has sufficient signal to be studiable, and determination of the effective sensitivity limit to be that which saturates the telemetry, and which provides a viable lower signal. Combining all these con-straints sets the limit near V = 20 mag, resulting in an estimated number of somewhat over one billion targets.
The broad-band photometric field provides multi-colour, multi-epoch photometric measurements for each object observed in the astrometric field, for chromatic correction and astrophysical analysis. Four photometric bands are implemented within each instrument.
4.4. Spectrometric instrument
A dedicated telescope, with a rectangular entrance pupil of 0.75× 0.70 m2, feeds both the radial velocity
spectrom-eter and the medium-band photomspectrom-eter: the overall field of view is split into a central 1◦×1◦devoted to the radial ve-locity measurements, and two outer 1◦×1◦regions devoted to medium-band photometry. The telescope is a 3-mirror standard anastigmatic of focal length 4.17 m. The mirror surfaces are coaxial conics. An all-reflective design allows a wide spectral bandwidth for photometry. The image qual-ity at telescope focus allows the use of 10× 10 µm2pixels
within the photometric field, corresponding to a spatial resolution of 0.5 arcsec.
The radial velocity spectrometer acquires spectra of all sufficiently bright sources, and is based on a slit-less spectrograph comprising a collimator, transmission grating plus prism (allowing TDI operation over the entire field of view) and an imager, working at unit mag-nification. The two lens assemblies (collimating and fo-cusing) are identical, compensating odd aberrations in-cluding coma and distortion. The dispersion direction is perpendicular to scan direction. The overall optical lay-out is shown in Fig. 5. The array covers a field height of 1◦. Each 20× 20 µm2 pixel corresponds to an angular
sampling of 1 arcsec and a spectral sampling of approxi-mately 0.075 nm/pixel. The focal plane consists of three CCDs mechanically butted together, each operated in TDI mode with its own sequencing, providing read-noise as low as 3 e− rms with the use of a dedicated “skipper-type” multiple destructive readout architecture with 4 non-destructive readout samples per pixel.
+Z +X +Y +Z +X +Y 403.23 mm 71.43 mm
Fig. 5. Optical configuration of the spectrometric instrument. The left figure shows the overall telescope design, while the right
figure shows details of the spectrograph optics
4.5. Science data acquisition and on-board handling
Preliminary investigations have been carried out to iden-tify the minimum set of data to be transmitted to ground to satisfy the scientific mission objectives; to identify some on-board data discrimination compression principles able to provide the targeted data compression ratio; to assess the feasibility and complexity of implementing such com-pression strategies and related algorithms on board; to assess the resulting compressed data rate at payload out-put, which are used for the sizing of the solid state memory and communication subsystem; and to derive preliminary mass, size, and power budgets for the on-board process-ing hardware. For estimatprocess-ing telemetry rates (Table 6), a specific spatial sampling of the CCD data has been as-sumed. This sampling is not yet optimised and final, but represents a useful first approximation.
The instantaneous data rate will primarily fluctuate with the stellar density in each of the three fields of view, which scale with Galactic latitude. On-board storage will store a full day of observation for downlink at a higher rate during ground-station visibility. Including overhead, the total raw science data rate is roughly a factor 7 higher than the mean (continuous) payload data rate foreseen in the telemetry budget (∼1 Mbit s−1). Data compression will reduce this discrepancy, but there remains roughly a factor two to be gained either by smarter CCD sampling, or by increasing the link capacity.
4.6. CCD details
CCD detectors form the core of the GAIA payload: their development and manufacture represents one of the key challenges for the programme. In the present study, con-sideration was given to the requirements on electro-optical behaviour; array size; buttability; pixel size; bright star handling; serial register performance; output amplifiers; power dissipation in the image zone, serial register, and output amplifier; trade-off between QE and MTF; photo-response non-uniformity; dark current; conversion factor
and linearity; charge handling capacity per pixel; charge transfer efficiency in the image zone and serial register; minimization of residual images; anti-blooming efficiency; and packaging. The present baseline design is summarised in Table 7.
For astrometric use CCD accuracy depends essentially on the integral of QE×MTF over the wavelength band. CCD MTF must be optimised in parallel with the QE. The QE and MTF values for the CCD optimised for the astrometric field have been used in the detailed astromet-ric accuracy analysis. The pixel size, nominally adopted as 9× 27 µm2 for the astrometric field, is an important
design parameter. A smaller pixel size would decrease the telescope focal length, as well as the overall size of the as-trometric focal plane assembly, and consequently the over-all size of the overover-all payload. However, such devices pro-vide worse performance in a number of other areas. The trade-off between QE, MTF, and charge-handling capac-ity results in design reference values, and bread-boarding activities are underway to verify these performances in detail.
A “worst-case” star density, corresponding to about 2.8 106 stars per square degree (about 19–20 mag in
Baade’s Window) has been used in a detailed analysis of CCD performance. The total noise per sample includes contributions from the CCD read-out noise at the relevant read frequency, analog-to-digital conversion noise, and the video chain analog noise.
Table 6. Average stellar flow in the various fields of the astrometric and spectrometric instruments, and the resulting average
telemetry rates. A limiting of G = 20 mag is assumed for the astrometric instrument (AF and BBP) and for the medium-band photometer (MBP), and G = 17 mag for the radial-velocity spectrometer (RVS). It is assumed that each sample represents 16 bits of raw data. The resulting raw data rates are before compression and do not include overhead
Parameter Astro-1 and 2 (per instrument) Spectro
ASM3 AF01–16 AF17 BBP SSM1 MBP RVS
Limiting magnitude, Gmax [mag] 20 20 17
Average star density, Ns [deg−2] 25 000 25 000 2900
TDI integration time per CCD, τ1 [s] 0.86 3.0 30
Field width across scan, Φy [deg] 0.66 1.0 1.0
Star flow through Φy, f = NsΦyω [s−1] 550 833 97
Number of CCDs along scan, NCCD 1 16 1 4 1 14 1
Solid angle of CCDs, Ω [deg2] 0.019 0.302 0.019 0.077 0.100 1.400 1.000 Number of stars on the CCDs, NsΩ 473 7568 473 1892 2500 35 000 2900
Readout rate, R = NsΩ/τ1[s−1] 550 8800 550 2200 833 11 662 97
Samples per star read out 25 6 30 16 or 10 42 14 930
Samples per star transmitted, n 25 6 30 10 42 8 930
Raw data rate, 16nR [kbit s−1] 220 845 264 352 560 1494 1443
Raw data rate per instrument [kbit s−1] 1681 3496
Total raw data rate [kbit s−1] 2× 1681 + 3496 = 6858 Specific laboratory experiments, using the 13 µm pixel
EEV device CCD42-10 in windowing mode, non-irradiated as well as irradiated at doses of up to 5 109 protons
cm−2, have been conducted in TDI mode, using different illumination levels, and at different CCD operating tem-peratures. Although not fully representative of the flight configuration, and while not yet fully evaluated, these ex-periments have demonstrated that the targetted centroid-ing accuracy appears to be achievable.
A key parameter for achieving a high degree of repro-ducibility is the Charge Transfer Efficiency (CTE) of the CCD. In the present context it is more convenient to dis-cuss the Charge Transfer Inefficiency (CTI) ε = 1− CTE. Very few charge carriers are actually lost (through recom-bination) during the transfer process; rather, some carri-ers are captured by “traps” and re-emitted at a later time, thus ending up in the “wrong” charge packet at the out-put; if short-time constant processes dominate, the main effect observed is that of image smearing. The CTI has an effect both on the photometric measurement (by reducing the total charge remaining within the image) and the as-trometric measurement (by shifting charges systematically in one direction). CTI during parallel transfer is particu-larly critical, since it affects the astrometric measurements in the direction where the highest precision is required, i.e. along the scan. In addition, CTI is worse along-scan due to the lower transfer rate. The magnitude of the problem can be crudely estimated as follows: assume a constant fraction ε of the charge is left behind while the fraction 1−ε flows into the next pixel. The expected centroid shift is ' Nε/2 pixels. The CCDs in the astrometric field of GAIA have N = 2780 pixels of size 9 µm along the scan.
Assuming ε = 10−5 results in a centroid shift of 125 nm or about 500 µas.
More careful appraisal of the CTI effects on the astro-metric accuracy show the effect after calibration is negli-gible for the the undamaged (beginning-of-life) CCD, but potentially serious for the degraded performance that may result after significant exposure to particle radiation in or-bit. Although most of the CTI effects can be calibrated as part of the normal data analysis, stochastic effects related to the charge losses can never be eliminated by clever pro-cessing. Extensive laboratory experiments are underway to quantify the amplitude of these residual effects.
4.7. Payload summary
In summary, the GAIA payload comprises the following elements:
(a) two identical astrometric telescopes: − fully-reflective 3-mirror SiC optics, − separation of viewing directions: 106◦,
− monolithic primary mirrors: 1.7 × 0.7 m2,
− field of view: 0.32 deg2,
− focal length: 50 m,
− wavelength range: 300–1000 nm, − 4-colour broad-band photometry, − operating temperature: ∼200 K,
